1951: Chapter 21 – Indirect Fire

INTRODUCTORY NOTES1. The indirect fire unit is normally the group of two or more sections, because:-

(a) The gun position is not in view of the enemy, and therefore the concealment and control of four guns is possible.

(b) Indirect fire is usually employed at the longer ranges, when the fire of four guns is desirable to produce the requisite volume.In indirect fire positions, the normal distance between the guns is 15 yards.

2. The opening of fire rapidly and effectively, by indirect means, depends on accuracy in the use of instruments and minute precision in drill. Such accuracy and precision can be attained only by a high standard of training and frequent practice.

LESSON 109.- SYSTEM OF INDIRECT FIRE CONTROL
A – INSTRUCTOR’S NOTES

Aim

3. To give a general picture of the system of indirect fire control, before considering each part in detail.

Class and instructors

4. One officer instructor and not more than 30 in the class.

Periods

5. One 45-minute period, lecture.

Stores

6. Blackboard and chalk.

Preparation

7. Draw the diagrams given below on the blackboard.

B – CONDUCT OF LESSONApproach

8. Explain the introductory notes and give the aim of the lesson (see para 3 above).

Pivot gun

9. State that No. 1 gun, the righthand gun, is always used in indirect fire as the basis for calculations. It is known as the pivot gun.

Zero line

10. Explain that to obtain direction in indirect fire, an object is selected in the centre of the arc. The line from No. 1 gun to this object is called the zero line. Once the pivot gun has been laid on this line, the group commander, by measuring the angle between the zero line and a target and ordering the pivot gun to swing through this angle, can get the gun laid on the target. To ensure accurate measurement of switches, the object selected for the zero line should be easily recognisable and clearly defined.

As the object selected for the zero line, will rarely be visible from the gun position, the line is prolonged into the gun position by means of zero posts (see Fig 42). If the pivot gun is mounted so that it is laid directly through the zero posts, it will then be laid on the zero line.

Paralleling

11. Explain that when the pivot gun has been placed on its zero line, the remaining guns are paralleled to it. The direction dialsand drums on all guns are then set at zero and lensatic sights aligned on the aiming post. If the guns are now switched through the same angle, they will still remain parallel, and, as the gun frontage is normally 45 yards, their lines of fire will cover 45 yards (see Fig 43 below).

At any time, the guns can be put back on their zero lines by resetting direction drums and dials at zero and tapping the guns until their lensatic sights are back on the aiming posts.

Elevation

13. When firing indirect, guns cannot be laid directly on the tagrte. To place the elevation on the guns to hit the target, two components are required.

(a) The range gun-target.- The range is usually determined by rangefinder or map. If by rangefinder, it will probably be taken from the OP. The group commander must therefore remember to increase it by the distance between the gun line and the OP. The range is corrected for climatic conditions before being placed on the range-drum of the dial sight.

(b) The angle of sight to the target.- This is measured by the group commander with the director from a point on the crest in front of the gun line. The angle of sight from the crest is assumed to be the same as from the gun line. The angle of sight is placed on the angle of sight drum of the dial sight.

The bubble is now levelled. This has the effect of placing the tangent angle plus or minus the angle of sight, ie, the quadrant angle on the gun.

Crest clearance

14. Tell the class that on all occasions on which an indirect position is occupied, it must be ensured that the guns will clear the crest. This is done by comparing the lowest elevation required in engaging the target, with the lowest elevation that will allow the cone of fire to clear the crest.

Direction of guns

15. State that as the dial sight is not attached to the gun at the point at which the gun pivots, the dial sight moves in an arc when the gun is swung through an angle. To avoid inaccuracies resulting from this, the guns must be mounted approximately in the direction of the zero line. As the angle through which the guns will then swing to bring them parallel on their zero lines will be small in consequence, the movement of the dial sight will not affect accuract. If, however, the guns had not been mounted in the approximate direction of the zero line, an appreciable error would appear in paralleling (see Fig 44 below).

Staggering of guns

16. Describe how when guns are mounted for indirect fire, they are not in a straight line but staggeredso that the dial sights of Nos. 2, 3, and 4 guns can be seen from the pivot gun.

Conclusion

17. Questions to and from the squad.

18. Sum up main points.

LESSON 110.- FIRE ORDERS, INDIRECT
A – INSTRUCTOR’S NOTES

Aim

1. To teach the sequence and layout of an indirect fire order.

Class and instructors

2. One officer instructor and not more than 30 in the class.

Periods

3. One 45-minute period, lecture.

Stores

4. Blackboard and chalk.

Preparation

5. Write the sequence of a fire order on the blackboard.

B – CONDUCT OF LESSONApproach

6. Give the aim of the lesson (see para 1 above).

Sequence

7. Point out the sequence of an indirect fire order:-

Angle of switch (including wind allowance if necessary).

Elevation (including wind allowance if necessary).

Load.

Right and left ….. taps.

Rate of fire (if other than normal).

Fire.

Explanation of headings in fire order

8. Explain each heading of the fire order as under:-

(a) Angle of switch.- This will be given out as a switch right or left of zero. If a correction for side wind is necessary, it will be added to, or subtracted from, the switch before the latter is given out. Angles of switch are given to the nearest 10 minutes.

Examples:-“All, one two degrees two owe minutes left of zero”

“All, three degrees five owe minutes right of zero”

(b) Elevation.- If a correction for atmospheric conditions or for wind is necessary, it will be added to, or subtracted from, the range before the latter is given out. Angles of sight are given to the nearest five minutes.

Examples:-“All, one eight fifty plus three owe minutes”

“All, one six hundred minus four five minutes”

“All, one seven hundred, angle of sight zero”

“Nos 1 and 2 guns, one five fifty plus three owe minutes, Nos 3 and 4 guns, one five fifty plus two owe minute”

(c) Right and left ….. taps.- The number of taps varies with the width of the target. Nos. 1 and 3 guns will always tap to the right first; Nos. 2 and 4 guns to the left.

Orders during a shoot

9. Corrections to direction will be given out in the form:-

“All, left three owe minutes”

Corrections to elevation will be given out in the form:-

“All, down fifty”
“All, up one hundred”

There orders may be given out verbally or by signal.

Duties of senior section commander

10. During the issue of indirect fire orders, the senior section commander is responsible for:-

(a) Repeating back all fire orders to the group commander before passing them on to the guns.

In Fig 45, if the angles B can be made equal to the angle A, then the line Y is automatically parallel to the line Z. This mathematical principle is applied in paralleling machine guns as follows:-

No. 1 gun is laid with the dial sight on its zero line. The angle A is then measured and ordered to No. 4 gun as “Left A degrees.” Left A degrees is set on the direction dial and drums of No. 4 gun, making an angle of A degrees between the barrel of the gun and the lensatic sight.

The gun is then tapped until the lensatic sight is aligned on the dial sight of No. 1 gun; No. 4 gun is thus made parallel to No. 1 gun.

The measuring of Angle A on the dial sight of No. 1 gun and the setting of it on the dial of No. 4 gun is shown in Fig 47 below.

The same process is then repeated for the remaining guns. For the sake of convenience the order of paralleling is Nos. 4, 3, and 2 guns.

Method of paralleling

8. State that based on the above theory, the sequence of paralleling is as follows:-

(a) Align the zero posts on the zero line.

(b) The guns are mounted with all dials and drums at zero and lensatic sights locked.

(c) The lensatic sight of No. 1 gun is aligned through the zero posts by moving the tripod or tapping the gun.

(d) The senior section commander measures the angle to Nos. 4, 3, and 2 guns from the zero line, reading the angles from the front pointer. He then closes the angle to make sure that No. 1 gun has not been disturbed whilst he has been measuring the angles from the other guns.

(e) The angles are placed on the dial sights, reading off the REAR pointer. The guns are tapped until their lensatic sights are laid on the dial sight of No. 1 gun.

(f) All drums are returned to zero. All guns are now on parallel zero lines.

(g) To maintain direction lensatic sights are now unlocked and aligned on the zero posts.

Rules of paralleling

9. Emphasize that if the following rules are remembered and followed, there can be no errors in paralleling:-

(a) Before paralleling begins, all dials and drums must be at zero and lensatic sights locked.

(b) The senior section commander reads off the FRONT pointer on the dial, the Nos. 1 off the REAR.

(c) When the guns have been paralleled, all drums and dials are again placed at zero.

(d) Once guns are parallel, they must not be tapped until the lensatic sight has been unlocked and aligned on the aiming post.

Practice

10. Revise the sequence of paralleling, by questions.

11. Demonstrate the planting of zero posts and practice the squad. Align No. 1 gun through the posts.

12. Demonstrate the duties of the senior section commander in measuring the switches to Nos. 4, 3 and 2 guns.

13. Practise the squad.

14. Move the squad to No. 4 gun and parallel it, using the angle measured.

15. Show, by looking through the tangent sights and noting where the line of sight cuts the ground, that the two guns are now, in fact, parallel.

16. Practice the squad in the whole operation of paralleling.

Conclusion

17. Questions to and from the squad.

18. Sum up main points.

LESSON 112.- TARGETS OF EQUAL OR LESS WIDTH THAN THE GUN FRONTAGE
A – INSTRUCTOR’S NOTES

Aim

1. To teach the method of engaging a target of not greater width than the gun frontage, by indirect fire.

Class and instructors

2. Lecture – One officer instructor and not more than 30 in the class. Practice – Squads under squad instructors.

Periods

3. One 45-minute period, lecture. One 45-minute period, practice.

Stores

4. Lecture – Blackboard and chalk. Practice – One director per two men and portable blackboard. Class require range tables.

Preparation

5. Draw the diagrams given below on the blackboard. For the practice, the instructor should select zero line and targets and prepare fire orders.

B – CONDUCT OF LESSONApproach

6. Give the aim of the lesson.

Direction

7. Explain that when engaging a point target or a target of less width than the gun frontage, the parallel lines of fire of the four guns should be so laid that they straddle the target. As the lines of fire normally cover a frontage of 45 yards, the line of fire of No. 1 gun should fall 22½ yards to the right of the centre of the target (see Fig 48).

To lay his guns for direction, the group commander should measure the angle between the zero line and the centre of the target with the director. When measuring angles, the director should as far as possible be mounted at a position in from of No. 1 gun. The group commander should then obtain from the VI table in the range graph what 22½ yards subtends as an angle at the range gun-target.

By subtracting this from the angle between the zero line and the centre of the target, he will obtain the switch required. This switch will be ordered “All …. degrees …. minutes left of zero.”

Should the target be on the right of the zero line, the angle subtended by 22½ yards at the range gun-target must be ADDED to the angle between the zero line and the centre of the target (see Fig 49 below).

8. Explain that if the target is of equal width to the gun frontage, then obviously the line of fire of No. 1 gun must be laid on the right end of the target. In this case, the group commander has simply to measure the angle between the zero line and the right end of the target and order this switch to the guns.

9. State that when measuring switches with the director, provided it is mounted not more than 30 yards to a flank or 100 yards in front or rear of the pivot gun, errors due to displacement will be negligible.

Tapping

10. Tell the class that to cover the gaps between the lines of fire and to cater for errors in direction, guns are tapped right and left one tap.

Elevation

11. State that elevation is ordered to the guns as laid down in Lesson 110. If no strike is observed, the combined sight rule is applied.

Practice

12. Revise the lesson by questions.

13. Indicate the zero line and a target and give the range.

14. The squad should now measure the switch and angle of sight and work out the fire order.

1. To teach the method of engaging a target of greater width than the gun frontage.

Class and instructors

2. Lecture – One officer instructor and not more than 30 in the class. Practice – Squads under squad instructors.

Periods

3. One 45-minute period, lecture. One 45-minute period, practice.

Stores

4. Lecture – Blackboard and chalk. Practice – One director per two men and portable blackboard. Class require range tables.

Preparation

5. Draw the diagrams given in Figs 50 and 51 on the blackboard. For practice, the instructor should select zero lines and targets and prepare fire orders.

B – CONDUCT OF LESSONApproach

6. Give the aim of the lesson (see para 1 above).

Direction

7. Explain that to engage a target of greater width than the gun frontage, the system is to lay the parallel lines of fire on the centre of the target and to cover the extra width of the target by tapping right and left.

To lay the parallel lines of fire astride the target, the group commander should proceed as in Lesson 112, para 7.

8. State that to discover the number of taps required to cover the extra width of the target, the group commander must measure the width of the target. From this, he should subtract the angle subtended by 45 yards at the range gun-target. This will give him the angle subtended by the extra width. Dividing this by two will give him the amount not covered by fire at each end of the target. This amount should be brought to the nearest number of taps. One extra tap should be added to cover errors in direction, and the total number of taps ordered to the guns in the form “Right and left ….. taps.”.

Example:-

Angular width target

= 2 degrees

45 yards at 2400 yards subtends

= 1 degree 4 minutes

Extra width

= 56 minutes

Amount not covered each end

= 56 / 2 = 28 minutes or two taps.

Add one tap to cover errors in direction …. three taps.
Order “Right and left 3 taps.”

9. Tell the class that four guns should not engage a target requiring more than right and left 4 taps. If a target exceeds this, it should be split up and be engaged as separate targets.

Elevation

10. State that elevation is ordered to the guns as laid down in Lesson 110. If no strike is observed, the combined sight rule is applied.

11. Explain that should a target of greater width than the gun frontage have a different angle of sight but the same range to each end, the angle of sight to the right end will be ordered to Nos. 1 and 2 guns and the angle of sight to the left end to Nos. 3 and 4 guns (see Fig 51 below).

Practice

12. Revise the lesson by questions.

13. Indicate the zero line and a target and give the range.

14. The squad should now measure the switch and angle of sight and work out the fire order.

15. Question the squad on the fire order and discuss it.

16. Repeat this process with other targets, including wind problems.

Conclusion

17. Questions to and from squad.

18. Sum up main points.

LESSON 114.- DEPTH TARGETS
A – INSTRUCTOR’S NOTES

Aim

1. To teach the method of engaging depth targets by indirect fire.

Class and instructors

2. Lecture – One officer and not more than 30 in the class. Practice – Squads under squad instructors.

Periods

3. One 45-minute period, lecture. One 45-minute period, practice.

Stores

4. Lecture – Blackboard and chalk. Practice – One director per two men and portable blackboard. Class require range tables.

Preparation

5. Draw the diagram given below on the blackboard. For the practice, the instructor should select zero line and targets and prepare fire orders.

B – CONDUCT OF LESSONApproach

6. Give the aim of the lesson (see para 1 above).

Direction

7. State that:-

(a) Targets, of equal or less width than the gun frontage, are engaged for direction as in Lesson 112.

(b) Targets of greater width than the gun frontage are engaged for direction as in Lesson 113.

Elevation

8. Explain that depth targets in indirect fire are engaged in a similar manner to direct fire. The group commander orders the range and angle of sight to hit half-way up the target. He then orders sufficient lifts in the form “All down fifty, All, up one hundred” to cover the depth of the target, guns being tapped right and left the number of taps ordered at each elevation.

9. State that the maximum dimensions of a depth target are 200 yards difference in the range to each end.

When engagagin depth targets on rising ground, the group commander should order extra elevations at his own discretion.

10. Demonstrate engaging different types of depth targets on the blackboard.

Practice

11. Revise the lesson by questions.

12. Indicate the zero line and a target and give the range.

13. The squad should now measure the switch and angle of sight and work out the fire order.

14. Question the squad on the fire order and discuss it.

15. Repeat this process with other targets, including wind problems.

Conclusion

16. Questions to and from the squad.

17. Sum up main points.

LESSON 115.- CREST CLEARANCE
A – INSTRUCTOR’S NOTES

Aim

1. To teach the method of determining whether the bullets will clear the crest in front of the guns.

Class and instructors

2. Lecture – One officer instructor and not more than 30 in the class. Practice – Squads under squad instructors.

5. Draw the diagram below on the blackboard. For practice, select an area with a suitable crest.

B – CONDUCT OF LESSONApproach

6. Give the aim of the lesson (see para 1 above).

7. State that the responsibility for ensuring that the bullets will clear the crest is primarily the group commander’s; the senior section commander has however certain responsibilities in this respect, and as a matter of drill, should always ensure that the bullets will clear the crest.

8. Explain that it is not always necessary or practicable for the group commander to resort to the measurement by instruments and calculation of crest clearances during his reconnaissance for the gun position. With practice it is usually possible to judge how far back the gun position can be without the risk of bullets failing to clear the crest. As a rough guide, if the group commander walks up the crest until he can just see the target and then sites the gun line three paces in rear, the guns should just clear the crest and yet the gun position be indirect to the target.

If however, the range is short and the slope steep, it is advisable for the group commander to determine the minimum quadrant angle and to compare it with the lowest quadrant angle and to compare it with the lowest quadrant angle that he may require to engage his target, before deciding on his gun line. The procedure for doing so is given below.

9. The senior section commander should always determine the minimum quadrant angle and report this to the group commander when reporting “Guns on zero line”.

Theory of crest clearance

10. Tell the class that in order to lay a gun so that it would hit the crest, the range to the crest would be placed on the range drum, the angle of sight to the crest placed on the angle of sight drum of the dial sight and the bubble levelled. If the gun was now fired, the lower half of the cone of fire would strike the crest (see Fig 52 below).

Therefore, to lay the gun so that will just clear the crest, the barrel must be lifted through the angle subtended by the lower half of the cone of fire at the range gun-crest.

Thus we have on the gun:-

(a) Tangent angle for the range to the crest.

(b) Angle subtended by half the cone of fire at that range.

(c) Angle of sight to the crest.

(a) and (b) are constant for any given range and when added together are called the crest clearance angle. Crest clearance angles for all ranges from 50 to 4000 yards are given in the range tables.

Thus the minimum quadrant angle (the lowest angle that can be placed on the gun which will ensure that the whole of the cone of fire clears the crest) is composed of:-

The angle of sight to the crest and the crest clearance angle for the range to the crest.

Procedure

11. State that:-

(a) Immediately the guns are in position, the senior section commander will order the No. 1 of the gun lowest down the crest to measure the angle of sight to the highest part of the crest over which the guns will fire.

(b) The senior section commander will ascertain the range to the crest by estimation or range-finder, look up the crest clearance angle for this range and add it to the angle of sight to the crest. He will then report this angle, the minimum quadrant angle (MQA), to the group commander, who will compare it with the lowest angle he will employ to engage the target. If the lowest quadrant angle is equal to or greater than the MQA the guns will clear the crest.

(c) If necessary, the group commander can calculation the MQA before the guns come into action, measuring the angle of sight to the crest with the director.

12. Tell the class that after the guns have been laid for direction and elevation the Nos. 1 will check for crest clearance as laid down in Lesson 77.

Practice

13. Revise the lesson by questions.

14. Explain and demonstrate calculating the MQA using a gun to measure the angle of sight.

15. Practice the squad.

16. Explain and demonstrate calculating the MQA using a director to measure the angle of sight.

17. Practice the squad.

18. Practice the squad in comparing the MQA with the lowest quadrant angle.

Share this:

Like this:

LikeLoading...

Vickers MG Collection and Research Association

This page is published by the Vickers MG Collection & Research Association, a not-for-profit company, limited by guarantee, registered in England, Company Registration Number 07855202 – www.vickersmg.org.uk